(1) Field of the Invention
This invention relates to abandonment and recovery or ‘A&R’ procedures used in marine pipelaying, in which a floating vessel such as a barge is used to lay an offshore pipeline.
The invention has particularly advantages when used with J-lay pipelaying equipment and it will be described in that context, but it is not necessarily limited to J-lay applications.
The J-lay technique is suitable for pipelaying in deep water. It involves welding together successive pipe sections or ‘joints’ in an upright orientation in a J-lay tower on a pipelaying vessel. The resulting pipe string is launched downwardly into the water as it is formed. The pipe string adopts a single bend as it nears the seabed to lend a J-shape to the pipe string extending between the vessel and the seabed—hence ‘J-lay’.
J-lay is necessary in deep water because the pipe string with attached accessories extending from the pipelaying vessel to the seabed is extremely heavy, typically weighing hundreds of tonnes. To avoid buckling, the pipe string must bear that weight in tension, suspended from a holding device on the J-lay tower.
(2) Description of Related Art
An example of a J-lay pipelaying vessel is the Applicant's derrick lay barge Acergy Polaris. The operation of Acergy Polaris during pipelaying will now be outlined with reference to FIGS. 1 and 2 of the drawings. It should be noted that this example is given simply to put the invention into context and so does not limit the scope of the invention. In those drawings:                FIG. 1 is a side view of a J-Lay tower on a barge; and        FIG. 2 is a perspective view of an erector arm loading a double joint into the tower of FIG. 1.        
Referring to FIGS. 1 and 2, the J-lay tower 10 of the barge 12 is supplied with double joints 14 of pipe made onshore, which are stored horizontally on the deck 16. As required, the double joints 14 are lifted successively in horizontal orientation from the deck 16 to a tower entry level 18 using a pipe elevator system 20 best shown in FIG. 2. Here, a double joint 14 is loaded into a pivoting erector arm 22, which upends the double joint 14 into an upright orientation and passes it over to a tower handling system comprising a tensioner 24. The double joint 14 is then lowered and aligned with the pipeline end held in a support bushing at a work station 26 on the tower 10.
The double joint 14 is welded to the pipeline end at the work station 26 before the load of the pipe string is transferred from the support bushing to the tensioner 24 near the top of the J-lay tower 10. The completed pipe string is then lowered down to the support bushing for the addition of the next double joint 14. The tensioner 24 and the support bushing alternate to grip the pipeline end, interacting in a so-called ‘hand-over-hand’ manner.
‘Abandonment and recovery’ refers to the procedures of laying down and retrieving a pipeline end from the seabed. Those procedures are necessary during normal pipelaying start-up and termination. They are also necessary whenever pipelaying must be interrupted and resumed. For example, the pipelaying vessel may suffer a critical breakdown. More commonly, pipelaying is interrupted due to deteriorating weather conditions, which may stress the pipeline and reduce its fatigue life as the pipelaying vessel rolls and pitches in a rough sea or if it has difficulty remaining in position due to winds and currents. In such cases, the pipelaying vessel may need to abandon the pipeline end and leave the work area. When the problem that caused abandonment has passed, the vessel will return later to recover the pipeline end and resume pipelaying.
Abandonment involves attaching a cap to the pipeline end, typically a pipeline end terminal or ‘PLET’. A shackle is attached to a hook on the PLET to secure a wire running through a winch on the pipelaying vessel, and tension is transferred from the tensioner of the J-lay tower to the winch. The winch then lowers the PLET into the sea until the pipeline and PLET rest on the seabed. The wire is then detached from the hook of the PLET, for example using a remote-controlled linkage or an ROV, and is retracted by the vessel for storage.
A recovery buoy is attached to the PLET during abandonment, enabling the PLET and the pipeline end to be located and retrieved during a subsequent recovery procedure. In essence, the recovery procedure is the reverse of abandonment as the wire is reattached to the PLET, typically using an ROV, and the PLET with the attached pipeline end is winched back up to the pipelaying vessel for pipelaying to resume.
It may be possible in some situations for the pipelaying vessel to remain on station above the abandoned pipeline, for example while riding out a period of bad weather. In that case, the wire may be kept attached to the PLET resting on the seabed until the bad weather has passed. This eases the recovery procedure considerably.
Traction (capstan) winches are generally used in A&R procedures to handle the high top tensions characteristic of deep-water pipelaying. Such winches require a continuous length of wire, generally of steel. That wire must be of large diameter—typically 70 mm to 120 mm—to support the great weight of the pipe string that extends above the seabed as the pipeline end nears the surface. Obviously, the wire must also be very long: generally several thousand meters long. Consequently, the wire itself may weigh in excess of 300 tonnes, and it takes up a great deal of space on the vessel when not in use. It also requires a large, powerful and hence expensive winch.
Until recently, it was not possible to manufacture continuous lengths of wire of the necessary diameter. Indeed, A&R wires remain a high-cost item. They are also susceptible to damage, particularly in the corrosive marine environment, and so have a limited life. If damaged, the whole wire may have to be down-rated or rejected; this makes it advisable for the pipelaying vessel to carry a spare wire but this, of course, doubles the problems of high cost and storage space.
Multiple winch and wire systems have been proposed in an effort to mitigate the problems of using A&R wires. An example is disclosed in U.S. Pat. No. 7,507,055, assigned to Subsea 7 Ltd. This recognises that A&R operations do not always take place at extremes of depth and that those operations can be handled more conveniently, where possible, with a smaller-capacity wire and winch than with a larger-capacity wire and winch. Consequently, pipelaying vessels are often equipped with both larger-capacity and smaller-capacity wires and winches.
These differently-rated wires and winches may be used together or successively. For example, when abandoning a pipeline, the larger-capacity wire and winch may be used to lower the pipeline end to an intermediate depth at which the top tension reduces to an extent that the load can be transferred to the smaller-capacity wire and winch. The higher-capacity wire can then be disconnected from the pipeline end and retracted to the pipelaying vessel. The smaller-capacity wire and winch then takes over to lower the pipeline end the rest of the way to the seabed. This means that a shorter length of larger-diameter wire is required on the drum of the higher-capacity winch, reducing space requirements and potentially also cost. However each wire remains vulnerable to damage and if spares are kept on board for both wires, the pipelaying vessel must accommodate four wires and not just two. This consumes space and reduces any cost advantage.
It has been proposed to use complete single lengths of synthetic rope with traction winches for A&R purposes but that is not yet a proven technology. It is also noted that any damage to any part of a continuous rope may lead to the entire rope being down-rated or rejected, like a wire.
It is against this background that the present invention has been devised.